Method and apparatus for transfer of a reticle pattern onto a substrate by scanning

ABSTRACT

A system and method for transferring a reticle (201) pattern to a substrate image (203) by scanning. The reticle is placed between an illumination system (118) and the projection lens (117). The substrate is located below the projection lens. A loading system, wafer adjustment system, and focusing system are also disclosed.

This is a Division of application Ser. No. 07/788,146 filed Nov. 4, 1991now U.S. Pat. No. 5,298,939.

BACKGROUND OF THE INVENTION

The present invention relates to the field of photolithographictechniques. More specifically, one embodiment of the invention providesan improved lithographic method for fabricating a multitude ofelectronic devices, including those fabricated on silicon wafers such asthose used for linear, power, and other applications. This method isalso useful for printing multichip modules, CCDs, diskdrive heads, etc.and smaller flat panel displays. Still more specifically, the presentinvention relates to a manufacturing method and apparatus wherein animage from a photomask or a reticle (hereafter called a reticle) isimaged onto a photosensitive substrate by scanning means.

Currently there are three types of machines used for printing variouspatterns in the manufacture of electronic devices. The first type ofmachine is called a contact or proximity printer. This is the oldesttype of machine. The reticle and substrate are in contact or closeproximity and are aligned to each other. A flood exposure illuminatorilluminates the reticle and thereby exposes the substrate. This machineis relatively low in complexity, and relatively low in cost, but has thelarge disadvantage that reticles are ruined after a certain number ofuses. Manufacturers who use these machines would like to switch to oneof the other two types of machines, except the cost is oftenunacceptable.

The second type of machine is commonly called a step and repeat camerabecause it moves to a specified location and prints a portion of thephotosensitive substrate and then moves to another location andtypically prints the same image on another portion of the substrate,repeating this process until the entire substrate is printed. Withinthis category, there are at least three different object to imagemagnifications used. Some of the machines manufactured by Nikon, Canon,GCA and others are called reduction steppers, and can print a portion ofthe final image at 1/5× or 1/10× magnification. The reticle area printedis equal to the image area divided by the magnification. The state ofthe art machines can now print image areas of up to 20 mm×20 mm at 0.45or higher numerical aperture. Another manufacturer of step and repeatcameras, Ultratech uses a 1× magnification. The Ultratech stepper canprint a field of at least 10 mm×30 mm at 0.40 numerical aperture. Onedisadvantage of the 2× or any non-1× step and repeat technology is theneed for precise magnification control. The step and repeat technologyrequires very precise alignment, which requires a very expensive stagewith a precise metrology system. All the step and repeat cameras arecomplex, expensive and very large, consuming a large area of expensivefloor space. The step and repeat cameras have been designed primarilyfor printing dynamic memory integrated circuits. This market requiresresolution down to sub-micron which in turn requires extreme performancefrom these machines.

The third type of machine is called a scanner. Silicon Valley Group(formerly Perkin-Elmer) and Canon both make scanners that print a 6-inch(150 mm) wide image at 1× magnification with resolutions down to about 1micrometer. These machines are very complex and expensive. See U.S. Pat.Nos. 3,748,015 and 4,293,186. The types of lens system described in thepatents consists of two mirrors and putatively forms an image of goodresolution in a circle of about half the diameter of the larger mirror.The lens configuration only uses a portion of the high resolutioncircle, giving these machines the title "Ring field scanner." Thereticle object is imaged on the substrate in the form of a circular arc.Printing is accomplished by moving both the reticle object and substratetogether, with the reticle object and substrate to be printed onopposing sides of the optical system. Although a symmetric unity or 1×magnification lens system does not necessarily have any inherentdistortion, the difficulty in manufacturing these optical systems causesthem to have a large amount of "manufactured" optical distortion. TheCanon MPA-600 and Silicon Valley Group PE700 can typically print anapproximately 6-inch wide path.

Both types of machines typically use off-axis alignment techniques whichrequire calibration and the use of more environmentally stablematerials. The by-product again is increased costs and decreasedproductivity.

From the above information it is seen that an improved system and methodfor fabricating substrates is needed.

SUMMARY OF THE INVENTION

An improved system and method for fabricating substrates is provided byvirtue of the present invention. The invention provides a means toexpose a substrate by transferring an image from a reticle object to animage substrate with the correct image orientation and overlay toprevious layers exposed onto the substrate. This process is repeated fornumerous substrates quickly with a small, inexpensive, and reliableapparatus.

According to one aspect of the invention, the reticle is full sized witha unity (1×) magnification image of the entire area to be exposed on thesubstrate. According to a preferred embodiment of the invention, theoptical system includes a small field non-inverting unity magnificationrelay lens system located between the reticle object and substrate, andan illuminator to cover the field of the lens. The process illuminatesthe transparent reticle from its backside (top). The lens system, whichis composed of two 1× or unity magnification relay lenses required toprovide an erect and non-inverted image, relays an image of the scannedreticle object from the front side of the reticle onto the face of thesubstrate.

According to a preferred embodiment of the present invention, the systemprovides two easily adjustable mechanisms to individually change bothilluminator and lens numerical apertures. This allows the resolution ofthe system to be limited to the resolution requirements as defined bythe users specific process. This adaptability reduces printed defectssmaller than the numerical aperture can provide.

According to a preferred embodiment of the present invention, a stagesystem is employed to hold the reticle and substrate and move them inunison relative to lens and illumination system. The stage will further,move the substrate into alignment and focus relative to the lens, beforetightly coupling the substrate and reticle together and moving them inunison. Image transfer is accomplished with a stage motion in a twodimensional serpentine pattern to transfer the image of the reticle ontothe substrate through the lens.

According to a preferred embodiment of the present invention thegeometry of the alignment system produces a signal that is zero atperfect alignment. The stage is servoed to that signal and, therefordoes not require a stage metrology system.

According to a preferred embodiment of the present invention, in orderto reduce the size of the machine, a two axis planar stage system isemployed.

According to still another aspect of the present invention, a substrateloader is included to allow for automatic exchange of the substrates toand from the substrate chuck. The loader mechanism transports asubstrate from a substrate storage cassette located inside the machine,to save additional space, to the flat finder in the case of siliconwafers and then to the substrate chuck in the lower portion of the stagefor exposure. After exposure the substrate is transported to a secondsubstrate storage cassette.

Focus adjustment is also provided. According to this aspect of theinvention, the lens is equipped with an air gauge sensor to measurefocus. The stage with reticle and substrate is moved to severaldifferent locations and local gap measurements are made with the airgauge. The substrate is then adjusted for best overall focus.

A further understanding of the nature and advantages of the inventionsherein may be realized by reference to the subsequent text and attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a front view of one embodiment of the invention; FIG. 1b is aside view of the invention; FIG. 1c is a top view of the lithographychamber--all illustrating the arrangement of the internal assemblies ofthe invention;

FIG. 2 is a more detailed view of the lithography assemblies;

FIGS. 3a and 3b are schematic illustrations of the internal features ofthe optics including projection lens, illuminator, and alignment system;FIG. 3c is an illustration showing how the field shape is defined,including a light pipe cross-section, a magnified image of the lightpipe at reticle 201, an aperture 323 cross-section and final printingfield size, and an overlay image of the light pipe by aperture 323;

FIGS. 4a to 4f illustrate alignment features of the invention in greaterdetail;

FIGS. 5a, 5b, and 5c illustrate the focus system in greater detail;

FIG. 6a is a top view cutaway of the system while FIG. 6b is a frontview, FIG. 6c is a side view, FIG. 6d is a view of the air bearing, FIG.6e is a cutaway of the reticle stage, FIG. 6f is a cutaway of thesubstrate stage, and FIG. 6g is the substrate chuck;

FIGS. 7a and 7b illustrate part of the loader mechanism in greaterdetail;

FIG. 8 illustrates the architecture of the control system;

FIGS. 9a to 9g are flow charts illustrating operation of the systemsoftware; and

FIGS. 10a to 10c illustrate the scanning intensity of various scans ofthe system on the substrate; and

FIG. 11 illustrates an embodiment of the invention in which the reticleand substrate are disposed vertically.

DESCRIPTION OF THE PREFERRED EMBODIMENTS CONTENTS

I. Overall System Description

II. Optical Features

III. Alignment

IV. Focus

V. Stage Assembly

VI. Loader Assembly

VII. Electronics

VIII. Operation

IX. Conclusion

I. Overall System Description

FIGS. 1a and 1b are overall drawings of the system. The system is shownwith the external enclosure panels removed to expose the internal parts.The system includes a touch sensitive input screen 101 for the user tointerface with the system, modify various settings, and the like.

Containers 103 and 104 (boats or cassettes) are provided within themachine for storage of substrates. A flat finder unit 105 is providedand serves to locate the flat on silicon wafers and other substrateswhich use that type of alignment aid. Automatic substrate loaderarticulators 106, 107, 108, 109 are used to move the unexposedsubstrates from the substrate cassette 104 to the flat finder 105 and/ordirectly to stage 116. Automatic substrate unloader articulators 110,111, 112, 113 are used to move the unexposed substrates from the stage116 to cassette 103. Articulator parts 111, 112, 113 are identical to107, 108 and 109, respectively, but are not shown.

A reticle, mounted on a reticle adapter frame, is loaded by the user onthe top of stage 116. The stage 116 is used to hold the reticle 201 andsubstrate 203 (see FIG. 2a) and move them together on platen 114,exposing the substrate with imagery from the reticle. A lens system 117is used to copy reticle pattern information to the substrate 203.Illuminator 118 is used for exposing the substrate with actinic light.

A user interface computer 102, which resides inside main computer 122,operates the user interface screen and provides input to the system'smain computer 122. Other electronics are also housed in the lowerportion of the machine 119, 120, 121.

Clean and temperature monitored air is provided by fan 123 and HEPAfilter 124. As shown, air flow is from the normally cleanest area aroundthe system (the top rear) to the dirtiest (the front bottom) in theenclosed cabinet. The air preferably flows over the shelves in agenerally horizontal pattern over the shelves, which keeps thesesurfaces clean. To further facilitate a clean environment, all partswithin the lighography chamber have an electrically conductive andgrounded surface to reduce particle collection due to staticelectricity.

Lamp power supply 119 provides power to the lamp 301 located inillumination system 118. Machine driver electronics 120 include thestage drivers and other items like solenoid drivers. The high powersupply 121 supplies power to these functions. The controller computer122 directs the operation of the system based on input from the userinterface computer 102.

Umbilical 130 provides flexible electrical connections to the stage 116.Uninterruptable power supply 127 provides power to the computers 102 and122 in the event of a power failure. A pneumatic isolation system 115supports the platen 114 to isolate the platen/stage optics from outsidevibrations. Air pressure and vacuum pump 129 provides all necessarypressurized air and vacuum used by the machine, eliminating the need forexternal air and vacuum. As a consequence of the above, the only outsiderequirements are 2 kilowatts. The entire device is housed in frame 131.

Due to the overall architecture of the system, only access to the frontof the system is required for operation and normal maintenance. Thisfeature yields a small footprint and the immediate ability to locatesystems directly next to one another. It also provides for bulkheadmounting directly into a clean environment while being located in a lessclean environment. This optical lithography architecture also produces amaximum substrate to system area ratio of less than 20:1.

The combination of elements used and described within this systemproduces a total weight of less than 500 pounds.

The two-dimensional scanning lithography process outlined withinproduces an ability to image about 8 square inches of substrate persecond (assuming 100 mJ/cm² photosensitivity).

II. Optical Features

FIG. 2 illustrates a side view of the optical and mechanical features ofthe substrate exposure system in greater detail. In particular, thesystem includes a reticle 201 which has etched thereon a mask or patternof dark regions which block light from the illumination source so as toexpose selected regions of the substrate to light. The reticle may havea pellicle 202 to protect the object surface from contamination. Thereticle is oriented horizontally according to this embodiment. Below thereticle is the projection lens 117 which includes two telecentric 1×relays. Below the projection lens is a substrate 203 which is held on asubstrate chuck 204 by vacuum. Alignment of the substrate 203 to thereticle 201 is accomplished by the moving of an internal stage mechanism(the substrate stage, discussed below) in response to an alignmentsignal generated in the illuminator 118 (see FIG. 3b). Focus is measuredby an air gauge 205, and any errors in focus are adjusted by the Z,pitch, and roll motion of the substrate chuck. For purposes of thisdisclosure, the vertical direction in FIG. 2 is defined as the "Z"direction (otherwise referred to herein as "vertical"), while the leftand right direction in FIG. 2 is defined as "Y," and the direction intothe paper of FIG. 2 is the "X" direction. The yaw axis is defined asrotation about the Z axis; the pitch axis is defined as rotation aboutthe X axis; and the roll axis is defined as rotation about the Y axis.Illuminator assembly 118 is mounted above the reticle and is stationary.

FIGS. 3a and 3b show the machine optics which include projection lens117 and illuminator 118. FIG. 3a shows the projection lens, and FIG. 3bshows the illuminator with optics for alignment and camera viewing. Inthe illuminator system 118, an arc lamp 301 of sufficient power(preferably 100 to 350 watts) is located at the first focus ofellipsoidal mirror 302. Preferably, the lamp is electrically interlockedto its chamber lid, and is attached so it is removed when the lid isopened. Reflected light from the lamp, which has a spectral range fromabout 190 nm (nanometers) to about 1100 nm, at the first focus ofellipsoidal mirror 302, is reflected off mirror 303 onto internal lampchamber walls. The lamp preferably is in a three-wall chamber which iscooled with air flowing on the outside thereof. The light is thenreflected off partial reflecting cold mirror 305 which removes theunwanted infrared light from about 600 nm out to about 1100 nm andunwanted ultraviolet light by transmission to the mirror 303. There aretwo ultraviolet light rejection ranges used depending on the machineproduced. In one configuration, the larger resolution system, theultraviolet light rejected is from about 190 nm to about 300 nm. In theother configuration (a smaller resolution system) the ultraviolet lightrejected is from about 190 nm to about 360 nm. The unwanted transmittedlight is reflected off mirror 303 onto internal lamp chamber walls andis removed as heat. The lamp chamber is a three-walled structure tofacilitate management of the excess heat from the lamp. The structurehas three walls as seen in FIG. 2. Insulation is placed between theouter two walls so that the outer wall will be at ambient temperature.The inner two walls provide a large surface and air path length for heattransfer to the circulating forced air. Clean air from inside themachine lithography chamber is introduced through the bottom of the lampchamber into the inner cavity where it cools all internal elements. Itthen flows between the two inner walls where it absorbs additional heatfrom the inner wall. The hot air then passes out an insulated hightemperature hose to a vacuum source. The combination of filters reducesthe actinic light range (depending on which machine) to 300 nm to 440 nmor 360 nm to 440 nm. The lamp chamber assembly, including the lampchamber and optical elements 301-305, is attached to the rest of theilluminator 118 with precisely located pins and screws to that theoptical axis of the two halves of the illuminator align, but the halvesare easily separable.

An immersion lens 306 is used to focus the actinic light from mirror 305on the end of a trapezoidal shaped cross section fused silica light pipe307 which homogenizes the light to create a more uniform output. Thetransmitted, homogenized light at the output end of the light pipe 307is then coupled to the rest of the optical system by another immersionlens 308. An optical relay system including lenses 310, 311 and 316, andfolded by mirror 315 is used to magnify the image from the end of lightpipe 307 (f₁ in FIG. 3c) and refocus it on reticle 201. As shown in FIG.3c, the illuminator field f₂ is trapezoidal in shape and larger in sizethan the trapezoidal aperture f₃ 323 in the projection lens and is alsolarger than the designed field size of the projection optics. Thetrapezoidal image field shape is chosen so that the length of itsparallel sides are integer multiples of the 0.020-inch Sawyer motortooth pitch so that the stage may be whole-stepped during scanning andstill overlap the adjacent scanned portion of the substrate properly.

A three-position off/on/on shutter 313, which contains a green filterthat transmits the 546 nm mercury line and a hole, is used to eitherstop all light from being transmitted through the rest of the opticalsystem, or only allow 546 nm green light to be transmitted for use inbright field alignment, or pass all light used for substrate exposure.Mirror 315 reflects blue actinic light used for exposure andapproximately 50% of the light from 500 to 550 nm used for bright fieldalignment. This mirror also transmits light from 550 nm to 600 nm usedfor darkfield alignment. Mirror 315 transmits a small portion of theactinic light through lens 330 to exposure detector 331 for intensityregulation.

Variable aperture iris 312 defines the numerical aperture of theillumination system. For highest performance, including depth of focusand resolution as well as printed feature characterization, theillumination numerical aperture should be smaller than the projectionlens numerical aperture. This ratio, also known as partial coherence,typically should be near 0.45, but can be as low as 0.3 to as high as0.7. If the projection lens numerical aperature is changed, which isdone to reduce the printing of defects on the reticle smaller than thesmallest feature to be printed, then the illuminator numerical apertureshould be adjusted as well to an acceptable partial coherence ratio.Three-lens alignment is provided at the full NA of the projection lens.

Adjacent to iris 312 is a receptacle for holding a filter (not shown).Filters that can be used include neutral density and spectral bandpass.For example, to obtain the highest resolution with a lens, the usercould narrow the spectral bandpass to the most ultraviolet region of thelens bandpass.

The lens 117 is mounted on a rigid one-piece cantilever structure whichis rigidly mounted to the platen. The cantilever structure is made witha small length to width ratio to provide stiffness. The structure ismade of steel which is stiff, stable, has a relatively low temperaturecoefficient of thermal expansion, and is matched to the platen. Thisarchitecture allows the lens to be straddled by the stage, but stillhave sufficient stiffness so that it maintains the proper image planelocations. The illuminator is rigidly and precisely mounted to the lenscantilever structure in a manner to allow easy removal, to maintain arelationship of no relative motion between the platen, lens andilluminator system.

The projection lens is mounted horizontally and includes two identical1× magnification optical relays which is composed primarily of two Dysonlenses stacked together to form an upright image. The first opticalrelay includes prisms 317 and 322, lenses 318, 319 and 320, and mirror321. The second optical relay includes prisms 324, 329, lenses 325, 326and 327, and mirror 328. The transmitted light through the reticle fromthe illuminator is transmitted through prism 317, lens 318, lens 319,lens 320, and through aperature 206 to mirror 321. The light is thenreflected off mirror 321, back through aperture 206, through lens 320,lens 319, lens 318, and prism 322 to aperture 323.

An aperture 206, located on mirror 321 or mirror 328, is used to reducethe numerical aperture of the projection lens system. Reducing thenumerical aperture of the projection lens system, will reduce theresolution and increase depth of focus. If it is desired to print 5micron features, using the higher resolution lens with maximumresolution about 1.5 microns, then defects on the reticle of about 1.5microns and larger will be printed. If the resolution of the lens isreduced to 5 microns, then almost no defects smaller than that will beprinted, improving product yield.

Field aperture 323 is located at the image plane of the first 1× relayand defines the actual printing field of the lenses. The aperture servestwo purposes: 1) to reduce the allowable printing area to the designedfield of the relay optics which has the acceptable optical performance;and 2) to vignette any over illumination from reaching the substrate(see FIG. 3c). The intermediate image at aperture 323 is reversed leftto right, i.e., the long dimension of the trapezoidal image, but not topto bottom. The image from the first optical relay and aperture 323 arethe object for the second optical relay. The transmitted light throughthe aperture 323 from the first optical relay is transmitted throughprism 324, lens 325, lens 326, and lens 327 to mirror 328. The light isthen reflected off mirror 328 through lens 327, lens 326, lens 325, andprism 329 to the substrate 203. The final image will again be reversedleft to right with respect to the intermediate image at aperture 323,but not top to bottom. The net result is that the image of the reticleis copied to the substrate non-inverted and non-reversed, and is exactlythe same size.

One of two lens systems can be used depending on resolutionrequirements. The first lens tabulated is considered the lowerresolution lens, and the second lens is considered the higher resolutionlens. Even higher resolution lenses are possible as well.

A lens for lithography with resolution at least as small as 10micrometers using a wavelength range of 300 nanometers to 440 nanometersis tabulated below. All dimensions in the table below are shown inmicrometers.

    ______________________________________                                        Image Plane TO = 3.00   Air       FIG. 3a                                     Radii       Thickness   Material  Ref. No.                                    ______________________________________                                        R1 = plano  T1 = 26.67  Fused silica                                                                            317                                         R2 = plano  T2 = 1.70   Air                                                   R3 = 205.317                                                                              T3 = 13.756 Fused silica                                                                            318                                         R4 = 41.849 T4 = 1.239  Air                                                   R5 = 39.767 T5 = 5.943  Fused silica                                                                            319                                         R6 = 84.313 T6 = 0.929  Air                                                   R7 = 181.901                                                                              T7 = 44.995 Fused silica                                                                            320                                         R8 = 252.402                                                                              T8 = 111.936                                                                              Air                                                   R9 = 207.169            Mirror    321                                         ______________________________________                                    

Note that the previous lens and the lens shown below have an object tofirst surface working distance of 3.0 mm which is larger than patentedlenses of this type.

A lens for lithography with resolution at least as small as 1.5micrometers using a wavelength range of 360 nanometers to 440 nanometersis tabulated below. All dimensions in the table below are shown inmicrometers.

    ______________________________________                                        Image Plane TO = 3.00   Air       FIG. 3a                                     Radii       Thickness   Material  Ref. No.                                    ______________________________________                                        R1 = plano  T1 = 26.67  PSK3      317                                         R2 = plano  T2 = 1.100  Air                                                   R3 = 281.317                                                                              T3 = 11.975 FK5       318                                         R4 = 48.042 T4 = 1.100  Air                                                   R5 = 40.934 T5 = 14.284 F14       319                                         R6 = 68.722 T6 = 0.300  Air                                                   R7 = 89.857 T7 = 12.121 Fused silica                                                                            320                                         R8 = 54.631 T8 = 154.975                                                                              Air                                                   R9 = 235.845            Mirror    321                                         ______________________________________                                    

Although the numerical aperture of the higher resolution lens can bereduced using aperture 206 to print 10 microns or higher feature sizes,the throughput of the overall system would be reduced beyond what isacceptable. Therefore, a second lens, which is shown above, is used for10 microns resolution. Because it has a larger field size and a broaderspectral bandwidth, it has nearly the same throughput as the higherresolution lens.

FIG. 3c compares the light pipe cross-section f₁, the magnified image ofthe light pipe on the reticle f₂, the aperture 323 cross-section andfinal printing field size, and an overlay of the image of the light pipebeing vignetted by the aperture 323.

Two relay systems are used instead of a single relay system with a roofprism for three major reasons: 1) to allow for an intermediate aperturewhich reduces the required alignment accuracy of the illuminator to thelens; 2) to incorporate right angle prisms that are much easier toproduce than roof prisms; and 3) the field size created by a roof prismis diamond shaped and is much smaller than that produced by two back toback 1× relays and thus has reduced throughput.

Light for alignment from both the reticle 201 and substrate 203 istransmitted through lens 316, mirror 315 and then relayed throughbeamsplitter cube 334 by lenses 332 (two of these), 333 and 338 toalignment detector 337 for determining alignment of substrate toreticle. Lens 333 is an optical element used to correct astigmatismintroduced by mirror 315. Beamsplitter cube 334 allows some of the lightto be imaged by the other lens 338 to camera 339 for user viewing. Thealignment detector is mounted such that it is re-imaged to the center ofthe trapezoidal field. During the alignment process, the stage is movedso that a point on the substrate or reticle will be re-imaged atalignment detector 337 if it is moved to that location.

III. Alignment

FIGS. 4a through 4e illustrate the alignment technique. FIG. 4f showshow the darkfield alignment light source illuminates the substratetarget through prism 329 to provide darkfield alignment. An alignmentkey 402 on the reticle includes four clear rectangular windows set at 90degrees from each other against an opaque background 403 such as shownin FIG. 4a . These four windows are oriented at 45 degrees with respectto the X and Y axes of the machine. The rectangles are the referencepoints for alignment. The alignment detector 406 is a four-sectorphotocell 408 as shown in FIG. 4c, or if the light level is too low,four separate photomultipliers 410a to 410e, as shown in FIG. 4d. Thereticle key 402 is aligned to the alignment detector 337 by motion ofthe stage 116 as shown in FIG. 4d. On the substrate at the same relativeposition with respect to the reticle is a pair of crossed gratings 404,against a clear background 405 as shown in FIG. 4b. The substrate isaligned by moving the substrate stage 608 and substrate target 404 underthe reticle key 402 on the reticle as shown in FIG. 4e. When darkfieldlight from an illumination source 340 (see light source in FIGS. 4b and4f, preferably a non actinic source such as green LED's or greenilluminator output) is directed at the substrate alignment target asshown, the left and right edges of the gratings are illuminated and thediffracted light is visible through the windows 402. The substratetarget 404 is aligned in the X direction when the light intensities ofdetectors (FIG. 4c) A+B=C+D. The substrate target 404 is aligned in theY direction when intensities at A+D=B+C. FIG. 4f shows one of at leasttwo light sources in one of the preferred positions, which is mountedone on each side of the lowermost prism 329. Another preferred positionwould be with the light source 340 radiating into the hypotenuse ofprism 329 and onto the substrate target. In the first preferredposition, it is mounted at an angle so that its output shines throughthe side of the prism and is refracted downward and through the bottomof the same prism to illuminate the substrate alignment target. Theangle at which the light from the light source irradiates the substratetarget is larger than the numerical aperture of the projection lens, andis therefore working in darkfield mode. Darkfield has the advantage thatfeature edges is all that is seen by the alignment optics giving betteralignment information than brightfield illumination does.

The reflected light from the substrate target fully fills the entracepupil of the projection lens. This means that the alignment system usesthe full numerical aperture of the projection lens providing improvedsensitivity to alignment errors, which in turn should provide a higheraccuracy alignment than other methods used.

V. Focus

There are two focus sub-systems used in the machine. One is an air-gaugeprobe system which is mounted to the underside of the lens housing 117near the front 205. This air probe is used to gauge the mechanicaldistance between the lens and the substrate. This measurement is done atleast three places on each substrate to determine the mean Z distanceand the pitch and roll error of the substrate surface with respect tothe lens. It gauges an air gap in the range of 20 micrometer to 100micrometer with a precision of 1 micrometer. The architecture of the airprobe gauge system takes the form of a pneumatic Wheatstone Bridge withthe working air probe gap acting as the variable leg. Three air gapstructures with exactly the same mechanical configuration and materialsas the working air probe at its mid-range position are used as the otherthree legs of the bridge. This architecture eliminates the effects ofall ambient conditions and produces a sensitive and stable gauge.

The air-gauge can also gauge what fraction of its exit aperture iscovered. This feature is used to measure the position of the edge of thesubstrate, thereby determining the X, Y, and yaw position of thesubstrate with respect to the lens. Its precision in this mode isapproximately 10 micrometers.

The second focus sub-system is an optical gauge which provides opticalcalibration for the air-gauge probe. FIG. 5 illustrates the method anddevice for this system. This gauge measures the position of the trueimage focal plane of the lens system. This information is used toperiodically calibrate and remove any drift in the focus of the lens oroffset in the air-gauge focus system. Two fiducial marks are used forthis focus process. One mark 501 is located on the reticle stageadjacent to the reticle 201. This mark (FIG. 5b) is in the form of a 50%duty cycle one-dimensional reflective/transmitting grating with a pitchof twice the resolution of the projection lens. The "lines" in thegrating are clear (transparent) in a chrome background. The gratings areoriented at 45 degrees with respect to the X and Y axes of the machine.The second mark 502 is a 50% duty cycle reflective/absorbing gratingwith the same pitch as the first fiducial mark. The "lines" in thisgrating are reflective chrome in an absorbing background such asnon-reflective chrome. The second mark 502 is permanently located on aside of the substrate chuck 204 in the location which allows the lens117 to image one fiducial mark 501 onto the other 502. The two gratingsare oriented with their grating lines parallel to each other. Thesubstrate grating 501 has an area which is large enough to be sampled bythe air probe gauge 205. The top surface of this substrate grating islocated at the nominal focus height, i.e., the height of the top surfaceof a properly focused substrate. The substrate grating mounted on thesubstrate chuck 204 is moved in Z by the focus actuators 603 (in FIG.6), and is moved laterally by the substrate stage 608 relative to thereticle stage 602 which carries the reticle 201. This capability is usedto drive the substrate grating relative to the reticle grating in anoscillatory motion in the axis of the grating which has periodicity (seeFIG. 5b). The driving motion must be greater than one period of thegrating.

Light passes from the illuminator through the reticle fiducial. Theimage of the reticle fiducial 501 is imaged by the projection lens 117onto the substrate fiducial 502 which reflects a part of that image backthrough the lens system and into the alignment system to alignmentdetector 337. This technique of using the projection lens as part of thealignment optical path is known as "through-the-lens" alignment. Theamplitude of the light received by the alignment detectors variesperiodically as the phase is shifted between the two grating by theoscillatory motion of the substrate grating. The output is in the formof a triangular wave (FIG. 5c) when focus is best, changing to a loweramplitude sinusoidal wave when out of focus. FIG. 5b shows the substrategrating 502 being moved to the left in the figure with respect to thereticle grating 501. Pattern 503 of FIG. 5b shows the two gratings at 0degrees phase, where the clear area in the reticle grating is alignedwith the reflecting portion of the substrate grating. This correspondsto the top of the triangle wave and maximum signal output. Patterns 504and 506 show the gratings at 90 degrees phase and 270 degrees phase,respectively, which corresponds to the signal output being half of thetriangle peak. Pattern 505 shows the gratings aligned at 180 degreesphase which corresponds to the minimum signal output at the bottom ofthe triangle wave. The substrate chuck and therefore the substrategrating is moved in pure Z until the amplitude of the detected periodicsignal is at its maximum. That height, which is the height of the truefocal plane, will be transduced by the air probe gauge directed at thesurface of the substrate grating. That information will be usedsubsequently to focus working substrates.

V. Stage Assembly

FIGS. 6a to 6d illustrate the stage section of the device. FIG. 6a is atop view of the stage (cutaway section a--a). FIG. 6b is a front view,and FIG. 6c is a side view (cutaway section c--c). FIG. 6d illustratesthe air bearing 609 in greater detail (bottom view and cutaway sideview, section x--x). FIG. 6e is a top cutaway view (section e--e)showing the inside reticle stage with the substrate stage removed. FIG.6f is a top view of the substrate stage only.

One element in the stage system is the platen 114 which is a large,stiff, stable cast iron structure. The platen has a flat top surfacewhich is held horizontally in the machine and forms a plane on which thestage 116 travels (refer to FIG. 1). The top surface has teeth formed inits low magnetic reluctance material (such as iron) in a two dimensionalarray which allows the use of Sawyer motors (which provide low particleproduction) in the stage to propel the stage in X, Y, and yaw axes. Theplaten teeth act as the stator of a Sawyer motor. The top surface isfurther processed by filling the gaps in the teeth array with a materialthat has a high magnetic reluctance, that is hard and can be lapped,such as nickel or chromium, to form a solid surface on which airbearings may fly. A thin layer of additional material is added tocompletely cover all teeth to eliminate rust and crevices forparticulates. Magnetically loaded air bearings are used extensivelyherein due to their low particle production and high stiffness. The topsurface is then lapped very flat to allow the stage to move on it whileproducing a small Abbey shift between the bottom of the stage 116 wherethe substrate 203 rides, and the top of the stage where the reticle 201rides. The isolation system is composed of the heavy platten 114 and airbags 115. The moving plant of the air bags, which is the contraint planeof the stage system, is placed near the center of gravity of the stagesystem. The platen is constructed so the contraint plane is slightlybelow the top surface of the platen which is also the force plane of thestage system. This relationship dictates a minimum system motion causedby the perturbing forces of the stage motion. The stage is intentionallymade as light as practical, and the platen is made as heavy as practicalso that the ratio of the two masses is as high as possible. Thisrelation dictates that any forces applied to the stage are translatedinto an acceptably small platen acceleration. This cause and effect isimportant because any acceleration experienced by the platen producesacceleration and therefore displacement in the lens, illumination, andalignment systems which would degrade lithography performance.

The stage system element which transports the electrical and pneumaticneeds of the stage from the machine is the stage umbilical (FIG. 1a,item 130). The umbilical is connected to a rear vertical corner of theouter stage structure. The other end of the umbilical is connected to avertical post mounted to a rear corner of the top surface of the platen,where it then continues to the driver electronics 120 and pneumaticsources 129. The stage umbilical 130 includes two small diameterpneumatic hoses, one each for vacuum and high pressure air. These hosesare bonded together and attached to an electrical cable. The cable isformed of printed wire on a thin stiff plastic sheet such as Kapton orMylar which is wide enough to accommodate all the necessary electricalconductors side-by-side in a single layer. The cable is mounted withit's width disposed vertically. The cable has low mass and has lowstiffness in the plane in which the stage 116 moves to reduce theintroduction of mechanical noise to stage but has high stiffness in thevertical axis such that it will follow the stage without adverselyaffecting it's motion but will not sag and drag on the platen.

The stage 116 includes an outer box (reticle stage, FIG. 6e) 602 and aninner transport system (substrate stage, FIG. 6f) 608. The reticle stage602 includes a reticle chuck 604, three magnetically loaded air bearings609, three inter-stage couplers 606, three focus actuators 603,miniature solenoid valves 615 for controlling the pneumatic functions,and pneumatic reservoirs for vacuum and high pressure air 614.

The reticle stage structure 602 is formed of a high specific stiffnessmaterial such as aluminum, with thin solid walls to form a light butstiff structure.

The reticle stage 602 is supported on the bottom by three magneticallyloaded air bearings 609 which fly on the platen 114. The air bearingsprovide a stiff frictionless support for the reticle stage 602 andconstrain it to move strictly in the X, Y plane. Each air bearing, asshown in FIG. 6d, includes a replaceable air bearing surface 620, apermanent magnet 621, an electromagnetic coil 622, and a top cup 626 ofiron which completes the magnetic circuit for both the coil andpermanent magnet. The replaceable air bearing surface 620 is a thin flatpiece of hard material. In the face of the hard material, which will actas the bearing surface, there are three orifices 623 feeding three smallgrooves 624 which form three air bearing elements. They receive airthrough air inlet tube 625. The air bearing face 620 is attached to, butremovable from, the other parts of the air bearing assembly in case thesurface is scratched or the orifices plugged. Another part is thepermanent magnet 621 which is made of a high B permanent magnetmaterial. It takes the form of a pulling magnet with a small air gapbetween its poles and the platen which the air bearing flies on. It isoriented to exert it's force vector toward the platen through the centerof the replaceable air bearing surface 620 part so as to equally preloadeach of the three air bearing elements 624. The third part of the airbearing is an electromagnetic coil 622. This coil of high conductivitywire is oriented concentrically about the permanent magnet and as partof the same magnetic circuit as the permanent magnet. Itselectromagnetic and thermal properties allow the coil to be pulsed witha short high current to temporarily counteract the high force producedby the permanent magnet. This action allows the air bearing to beinflated to its proper flying height from a position in contact with theplaten. Another reason for the coil is that it may be energized with amuch smaller current which may be controlled to modulate the totalmagnetic loading force, and therefore the flying height of the airbearing.

The reticle stage 602 has a large opening in the top horizontal surfacewhich forms an exposable area through which light may pass through thereticle 201 and into the top of lens 117. Around the opening of thereticle chuck are kinematic mounting points and vacuum pads 617 tosupport and hold the reticle frame in the proper position in the largeopening in the reticle stage 602. The proper position is defined as theposition where the image of the bottom surface of the reticle iscoplanar with and at the nominal focal plane of the upper relay lens117. The reticle mounting frame provides two important features. First,it reduces the need to physically contact the reticle during theinstallation process. This in turn, reduces potential contamination andbreakage problems. Second, it provides a means to mount and use morecommon size reticles which are smaller than the maximum allowed bydesign.

The reticle stage 602 has a slot opening in its front surface 615through which the substrate is presented to the substrate chuck 204 bythe substrate loader arm 106. The slot opening is just large enough toallow the substrate on the loader arm to pass and allow clearance forthe arm to deposit or remove the substrate 203 onto or from the chuck204, and allow clearance for any relative motion between the stage 116,which is on an isolation system 115, and the loader, which is not.

The reticle stage 602 has a large opening 618 in its rear surfacethrough which the lens 117 intrudes and may scan the entire top surfaceof the substrate with its image plane located just below the lower prismsurface and simultaneously scan the entire bottom surface of the reticlewith its object plane located just above the upper prism surface. Thelarge opening is large enough to allow the lens to overlap the edge ofthe substrate on both sides by an amount slightly greater than half thelens width and to allow some clearance between the lens housing and thereticle stage side walls.

The reticle stage 602 also contains three focus actuators 603 mountedinside to the floor of 602 to support the substrate chuck 204 which setsand slides on their adjustable points 605 thereby forming a planenominally parallel to the reticle and platen but adjustable in height(Z), pitch (p), and roll (r). Each focus actuator is a standard small DCmotor with a gear box which drives an eccentric cam which forms the hubof a circular ball bearing. The focus actuator is mounted with its majoraxis horizontal such that when the motor is driven, the highest point ofthe outer circular edge of the bearing (which does not rotate) may be incontact with and move the substrate chuck in a vertical motion.

The reticle stage 602 also contains three inter-stage couplers 606mounted inside to the floor. The inter-stage couplers are assemblieswhich can be controlled to couple or de-couple the reticle stage 602 andsubstrate stage 608 to move in the X, Y plane dependently orindependently, respectively. Each stage coupler 606 is anelectromagnet/air bearing assembly. A corresponding steel couplerflexure 607 is mounted on the substrate stage 608 and forms the otherhalf of the coupling. When the electromagnet is deactivated the airbearing forces the surfaces of the electromagnetic/air bearing assembly606 and the coupler flexure 607 apart allowing the reticle stage 602 andsubstrate stage 608 to move relative to each other. When theelectromagnet is activated it overcomes the air bearing and forces thecoupler flexure surface tightly together with the electromagnetic/airbearing surface where friction disallows relative motion. controllingthe coupling unit with an electromagnet rather than with the air bearingpressure significantly increases both reaction speed and reliability.

The reticle stage 602 also contains two pneumatic reservoirs 614elastically mounted inside to the floor. One is a volume used to storevacuum, the other stores high pressure air. These reservoirs provide animmediately available low impedance source for the pneumatic functionsand allow the pneumatic hoses in the stage umbilical to be of smalldiameter since they only need to accommodate the long term needs of thestage.

The substrate stage 608 includes a structure with integral air bearings616, two sets of Sawyer motors 610A-C and 611A-B, a substrate chuckassembly 204, and three inter-stage coupling flexures 607.

The substrate stage 608 is formed of a high specific stiffness materialsuch as aluminum, with thin solid walls to form a light but stiffstructure to which the other elements are fastened. Formed in the bottomsurface of the substrate stage 608 are air bearings 616 adjacent to theSawyer motors which support the substrate stage 608 off the platen. Theair bearings are magnetically loaded toward the platen by the Sawyermotors and are located to support each of the two motor sets in Z, pitchand roll, to keep the motors from rubbing on the platen or bending thesubstrate stage structure and causing it to rub on the platen. The airbearings need not be extremely stiff so they have a relatively highflying height and are not replaceable as individual elements.

The Sawyer motors 610 and 611 are mounted to the substrate stagestructure 608 with their bottom surface coplanar with the air bearings609 on the bottom of the reticle stage 602. They fly on the platensurface 114 on integral air bearings and propel the stage relative tothe platen when the magnetic poles of each segment are controlled byappropriate electrical excitation.

The two sets of three motors are disposed on opposite sides of the stage(in X) with their major axis in line with the predominate stage axis(Y). Each set of Sawyer motors can move in X and Y and the two setsdriven differentially create a yaw motion. The motors are placed suchthat they produce force through the center of mass of the stage (in theX, Y plane) so as to not produce yaw torques when linear motion iscommanded.

The two sets of Sawyer motors includes three motor pairs, two pairs610A-D disposed to propel the stage in its predominant axis (Y), and onepair 611A-B to propel it in (X). Each motor contains a pair of motorsegments which are controlled with their phases in quadrature. Two pairsof Y motors are used to provide maximum agility and motion smoothness inthe Y direction which is the dominant scan axis. Only one pair of motorsis used for X motion to save weight and because only a small step ismade in X during exposure so the contribution to throughput by the Xstage motion is small.

The Sawyer motors are controlled with the two X motors in the systemreceiving the same excitation, the two Y motors in each set receivingthe same excitation, and the Y motor sets receiving oppositedifferential excitation to control yaw motion.

The excitation for the motors is a symmetric 50% duty cycle trianglewave of oscillating current which is symmetric in amplitude about zero.This excitation provides a piece-wise linear relationship betweenexcitation phase, and displacement or force.

The substrate chuck 204 provides a flat surface on which the substrateis placed so that it may be held flat and focussed. The chuck has a topsurface which is large enough to properly support the largest substrateintended for the machine. The chuck is made of a stable, high specificstiffness material such as Aluminum.

The chuck is coupled by flexures 612 to the substrate stage 608 suchthat it is rigidly constrained in the three axes of the horizontal plane(X, Y, yaw), but may freely move in the other three axes controlled bythe focus actuators (Z, pitch, roll). The substrate chuck ismagnetically loaded against the focus actuators to augment gravity andinsure intimate contact. The chuck is referenced to the reticle stagefor focus because the Sawyer motors bounce slightly in Z as they move inX and Y. This motion if coupled to the chuck would detrimentally affectimage focus.

The chuck has a top surface which is very flat, hard, electricallyconductive, and grounded. The areas on the bottom of the chuck where thefocus actuators contact and slide must also be very flat, smooth, lowstiction, and hard. The chuck could be plated with nickel or chromium.

The substrate chuck 204 (FIG. 6g) has a top surface which is coveredwith many shallow depressions 630 which are evacuated to hold thesubstrate flat and securely to the chuck, or alternatively vented toatmosphere to release the substrate from the chuck and eliminate anysliding friction. The depressions will be pneumatically linked togetherin zones. The innermost zone is intended to be covered by a smalldiameter round substrate. The inner zone includes all of the shallowdepressions 630 in the chuck which would be completely covered by thesmall substrate. The next zone 632 includes those depressions which whenadded to the inner zone 631 would be completely covered by a squaresubstrate whose side dimension is greater than or equal to the diameterof the inner zone. Each additional zone forms an increasingly largerzone which alternate in shape between square and round (633, 634, 635,636). Each of the zones are linked to a common pneumatic plenum throughseparate flow restricting orifices such that all zones may be controlledsimultaneously without a catastrophic loss of either vacuum or pressuredue to zones which are not covered by a substrate.

Attached to the substrate chuck and capable of protruding up through thecenter of the substrate chuck is a smaller pop-up chuck 613. The pop-uphas the ability to move vertically between two positions. This verticalmotion must be smooth and must not produce errors in position in anyaxis which the substrate loading system or the substrate alignmentsystem cannot accommodate. The down position is such that the topsurface of the pop-up chuck is below the top surface of the substratechuck. The up position is such that the bottom surface of the substrateis far enough above the top surface of the substrate chuck for thesubstrate loader arm to pick-up or deposit a substrate with appropriateclearance.

The pop-up chuck is just large enough in diameter to stably support asubstrate, and small enough such that the substrate on the substratechuck does not sag an appreciable fraction of the depth of focus of themachine's lens due to gravity in the center where the pop-up is.

The pop-up chuck also has four shallow spots in its flat top surfacewith which to hold a substrate with vacuum or vent to ambient pressure.

Other embodiments of the invention will be useful, particularly whenvery large (e.g., greater than about 8- to 12-inch) wafers are to beprocessed. For example, it will be desirable to fix the position of thesubstrate and reticle chucks, and move the illuminator and the lenssystem in synchronization. Further, it will be desirable with largesubstrates to orient the substrate and reticle in a vertical position(i.e., such that the wafer and reticle "hang" from the top portion ofthe system as shown in FIG. 11 which will prevent distortion due tosagging created by the gravitational pull on the wafer and the reticle.Substrate 1102 and reticle 1104 are shown oriented in vertical planeswith respect to the local gravity vector, and suspended from substratechuck 1106 and reticle mount 1108, respectively. During the lithographyprocess, lens system 1110 is disposed between substrate 1102 and reticle1104, with illumination system 1112 being adjacent reticle 1104,opposite lens system 1110 as shown. Relative motion is provided betweenthe reticle and substrate on one hand, and the lens and illuminationssystems on the other to implement the scanning pattern described herein.It is also desirable that substrates and reticles are stored andmanipulated in a substantially vertical orientation.

VI. Loader Assembly

FIGS. 1a and 1b also illustrate the loader mechanism. The system allowsfor the automatic transfer of substrates onto and off of the stage. Thefunction is necessary to improve throughput and reduce substratecontamination exposure. The loader is comprised of three main elements.They are the carriage unit 128, substrate load articulator (on rightside of machine) 106-109, and substrate unload articulator (on left sideof machine) 110-113 (111-113 not shown). The substrate load and unloadarticulators are mirror images of each other. The load articulator isidentified as such because of its access to the substrate prealign unit105. Each of these elements are connected to the machine frame 131. Toreduce vibrations that might be caused by the loader system and othersources, the platen 114, stage 116, and optics 117 and 118 are isolatedby isolators 115. This isolation allows the substrate load and unloadactivities to occur during substrate lithography, with no effect to thequality of that lithography. The substrate prealign unit 105 coarselyorientates substrates in yaw. The carriage unit 128 supports twosubstrate cassettes 103 and 104 and the substrate prealign unit 105previously mentioned. The interface between the carriage and thecassettes is configurable to allow for various size cassettes.

The carriage unit can be moved along the front to back (Y axis) onbearing guide ways 109 and 113 (lower). This motion is supported toallow the carriage unit to assume two special positions. The front mostposition, allows the user to easily access the substrate cassettes 103and 104. The back most position allows the substrate load articulator toaccess any of the substrates within the substrate load cassette or thesubstrate prealign unit. The back most position also allows thesubstrate unload articulator to access any of the positions within thesubstrate unload cassette. The substrate carriage is moved bytemporarily linking it to and moving the substrate load articulator106-109. This linkage is established through an actuator 125 thatengages into a receptacle socket 126 on the substrate load articulatorsY motor assembly.

The substrate prealign unit 105 is comprised of a substrate vacuum chuck702, shown in FIG. 7, which is attached to a motor underneath it thatallows a substrate to be rotated. The substrate 203 is held to theprealign chuck 702 by vacuum. This vacuum may be removed so that thesubstrate can be removed by venting the vacuum through the prealignvacuum solenoid valve. A prealign vacuum switch located pneumaticallybetween the vacuum chuck 702 and solenoid valve provides a means for thesystem to verify the presence of a substrate. Four linear arrays 700 ofoptical sensors provide feedback to locate and position the substrate inthe correct yaw orientation.

Each of the two substrate articulator arms (106 and 110) have twodegrees of freedom (Y and Z). The substrate load articulator Z axisguide bar 107 is transported in Y by two Sawyer motor assemblies 133that move along guide bars 108 and 109. The substrate articulator arms(106 and 110) are mechanically attached to Sawyer motor assemblies 133that move along guide bars 107 and 111, respectively. Each Sawyer motorguide bar in combination with the Sawyer motor assemblies 133 constrainsmotor motions to a single axis. FIG. 7a shows a typical loader guidebar/Sawyer motor assembly 133 interface in cross-section. Sawyermotor/air bearing 708 is propelled and positioned along a guide bar (oneof six) and fixes the assembly 133 in two degrees of freedom. Permanentmagnet/air bearing 710 that flies above the guide bar constrains Sawyermotor 708 in the other three degrees of freedom. The upper Y motionSawyer motor 133 is flexured to 107 to allow small motions in the Y andZ axis so that it does not fight the lower motor.

To insure that the substrate is not dropped or positioned incorrectly,vacuum is used to hold the substrate 203 on the substrate loader fork106, as shown in FIG. 7b, which is at the end of the substrate loaderarm. The substrate may be released by venting the vacuum through thesubstrate articulator loader arm solenoid valve. A vacuum switch locatedpneumatically between the fork vacuum channel and the fork solenoidvalve provides feedback to the system when a substrate is beingsuccessfully held by vacuum.

Each substrate arm also has an optical interrupter-type sensor attachedto it's end. The optical interrupter sensor is used for scanning acassette of substrates to determine the population and position of anysubstrate. In order to transfer substrates onto or off of the stage, thestage (and thus the platen) must be at a position in space which isknown by the substrate articulator controller.

The X, Y, yaw, Z coordinate space between the loader and stage arecorrelated through a calibration procedure. This calibration consists ofloading and aligning a special test substrate. The X, Y, yaw, Z offsetsrequired to align that substrate are then stored and used to preciselyload subsequent substrates.

VII. Electronics

FIG. 8 illustrates the electronic architecture of the system. A singlechassis 828 may house both of the computer controllers in the machine102 and 122. This chassis would be mounted to a drawer slide in thelower part of the machine for maximum accessability. Both the userinterface computer 102 and the machine controller computer 122 would bepersonal computer-type motherboards with 80386 microprocessors. The twocomputers communicate with each other through their respective serialports 840 and 852. Both computers are booted and have their main programin their respective floppy disc drives 844 and 856. Both computersderive their power from a common PC power supply 860.

It is the main task of the user interface computer 102 to control theuser interface elements 804: keyboard 864, user job file floppy discdrive 868, and the touch screen video monitor 101. The keyboard is notpermanently mounted to the machine, but is attachable to providemaintenance and process engineers access to special functions within themachine. It interfaces directly to the user interface computer board.The video monitor is driven by the user interface computer through avideo driver card 848. The touch screen function of the video monitor isdriven through another serial port 849.

The machine controller computer 122 controls the individual electricalactuator and sensor elements in the machine through a general purposeanalog/digital input/output printed circuit card 836. It's analogoutputs are used for functions like driving the focus actuator DCmotors. The analog inputs are used to sense the signals produced by thefocus gauge and the alignment system. The many single-bit digitaloutputs are used to control actuators like solenoid valves. Themultiple-bit digital output channels are used to control the Sawyermotor drivers. The digital inputs are used to sense the single-bitfunctions like pressure or vacuum switches.

The remote user interface system 804 is mounted on the front and top ofthe machine for best human interaction. As described previously, thissystem is driven by the user interface computer 102 mounted in the lowerpart of the machine in the controllers box 828. The floppy disc drive868 allows the user to input pre-programmed job-specific instructionsand process data. The video touch screen presents all necessaryinformation to the user. The touch screen 101 feature allows the user toselect machine operating modes and data screens.

The machine drivers 120 are all the high power actuator drivers for themachine. It includes Sawyer motor drivers for the stage and substrateloader, 888 and 880, respectively, and various power drivers for the DCfocus motors and solenoids 884. The loader motor drivers 880 operatestrictly in the whole step mode. They are switch mode bipolar currentamplifiers which produce the triangle wave current required by theSawyer motors. The stage motor drivers 888 are also switch mode currentamplifiers which produce triangle wave current. Additionally, the stagemotor drivers must be controllable in the micro-stepping mode to 12-bitprecision so that the 0.02-inch pole pitch stage motors can bemicro-positioned during alignment to 0.10 microns.

The machine drivers 120 are all powered by a set of large high powersupplies 121. These power supplies are mounted to the same drawer as themachine drivers and include a 24 VDC supply for the solenoids and a ±36VDC high current supply for the Sawyer motors. The high voltage isrequired to drive the required current at the high frequenciesassociated with the high stage speeds commanded during machineoperation.

The illuminator power supply 119 drives the mercury arc lamp used in themachine. The power supply has interfaces with the machine controller 836and the illuminator 820. The interface with the machine controller is toprovide the controller with lamp voltage and power information. Theinterface with the illumination system is through a shielded highvoltage cable to drive the lamp, and to the illumination detector 331used as feedback to control the lamp to a constant optical power.

The illumination electrical system 820 includes the alignment detectionfunction 337, the focus detection function 894, control of the shutterand the darkfield illumination devices 896, and the mercury arc lamp 301and illumination detector 331 mentioned above. The alignment detection337 is performed by a quad-cell photodiode. The four signals generatedby the alignment optical signal are amplified and sent to the machinecontroller I/O card 836 as analog signals. The focus detectionelectronics 894 includes differential amplifiers to sense the imbalancein the pressure transducer bridges, and buffer amplifiers to compare thesignals and drive the signals back to the machine controller I/O card836. The shutter driver and alignment illumination drive are simpleon/off drivers 896. The alignment illumination is a pair of greenlight-emitting diodes 340 used for illumination in the darkfieldalignment system. The illuminator detector 331 includes a photodiode andan amplifier to drive the signal back to the illuminator power supply119. The illuminator lamp 301 is a mercury arc lamp. It is driven fromthe illuminator power supply 119. The drive power involves relativelyhigh power and high electrical noise emissions, so the cable is wellinsulated and shielded. In the lamp chamber of illuminator 118, the lampdrive cable must also withstand the high temperature and ultravioletradiation environment. To facilitate maintenance and safety, the lamp isattached to the removable lid of the lamp chamber and includes aconnector mechanism for the other end of the lamp. When the lid isremoved, the lamp is also removed and both of the electrical connectionsare automatically broken.

The loader electrical system 816 includes Sawyer motors for Y and Zmotion on both sides of the machine, vacuum control for the substrateforks 106 and 110, rotation and vacuum control for the prealigner chuck702, and illumination and sensing electronics for the flat findersensors 700.

The stage electrical system 812 includes Sawyer motors for X, Y, and yawmotions 610A-D and 611A-B, three DC motors for the focus actuators 603,the solenoids used in the inter-stage couplers 606, drivers toraise/lower the pop-up chuck 613 in the substrate chuck 204 and controlits vacuum, solenoid valves to control the vacuum to the substrate chuck204, and the electromagnetic coils and actuators to control the highpressure air in the stage air bearings 609. Vacuum and pressure switcheswill also be required to sense the successful switching of vacuum andair pressure on the stage. Of the functions mentioned above, theactuators for the pneumatic functions are solenoid valves 604. All ofthe electrical signals pass through the stage umbilical 130.

VIII. Operation

During normal operation, substrate cassettes are exchanged in thefollowing manner. First, a command from the user is received to exchangesubstrate cassettes. The system continues processing any substratescurrently on the stage or the substrate load fork or prealign chuck. Anysubstrates on the stage are transferred to the unload cassette. Bothload and unload articulator substrate forks 106 and 110 are driven inthe Z direction to the substrate prealign station elevation. Both loadand unload articulators are driven toward the back of the machine (+Ydirection) to engage the carriage actuator 125 into receptacle 126 onthe carriage 128. Carriage 128 is pulled toward the front of the machine(-Y direction) by the articulators 107 and 111 to the cassette exchangeposition at the front of the machine. The user exchanges cassettes andacknowledges completion to the system through the user interface 101.The articulators 107 and 111 then push the carriage 128 back to itsoperational position. The carriage 128 releases the articulator driveactuator 125 from receptacle 126. Each of the articulators move to aposition such that their substrate forks 106 and 110 are just in frontof and below the lowest possible substrate in each cassette. Eacharticulator substrate fork then scans each of the two cassettes frombottom to top recording the presence of substrates.

Substrates are exchanged from the stage 116 in the following manner. Ifthere are no substrates on the load fork 106 or in the prealign station105, the load fork moves to just below the lowest substrate position inthe load cassette. The load fork vacuum is turned on and the fork beginsmoving up until vacuum is sensed. The fork then moves the substratetoward the front of the machine until it is clear of the load cassetteand carriage assembly. The fork then lowers the substrate to theprealign station elevation and then moves the substrate toward the rearof the machine and over the prealign vacuum chuck. The prealign chuckvacuum is turned on and the fork lowers the substrate until vacuum issensed. The load articulator fork vacuum is then turned off. Prealignmotor then rotates the substrate to locate and position the substratefor proper yaw orientation. Vacuum to load articulator fork is restoredthen load articulator is raised until vacuum is sensed at fork. Prealignvacuum is then released at prealign chuck and the substrate is drawntoward the front of the machine and out of prealign station.

If prealign, processing is complete and there is no substrate on thestage, the stage is moved to the load position. The reticle stage 602 isthen locked to the platen and the reticle and substrate stages aredisconnected. The substrate stage 608 is moved to its most forward(toward the front of the machine) and left position relative to thereticle stage 602. The substrate stage is then moved to the center ofthe opening in bottom of the reticle stage 602. The reticle andsubstrate stages are then reconnected by activating stage couplers 606and the reticle stage 602 is released from the platen. The stage pop-upchuck 613 is raised and the stage pop-up chuck vacuum is turned on. Thethree focus actuators 603 are positioned to their lowest point. Thestage then moves forward until the pop-up chuck is under the center ofthe substrate. The load articulator lowers the substrate until stagepop-up 613 vacuum is sensed and then removes vacuum from the loadarticulator fork 106. The stage 116 moves toward the rear of the machineto clear articulator fork 106. Stage substrate chuck vacuum is turned onand the stage pop-up chuck is lowered. After the stage substrate chuck204 vacuum is sensed the pop-up chuck 613 vacuum is turned off. Thesubstrate is leveled and focused by moving to a minimum of threepositions programmed by the user. At each of these points, thesubstrate/lens gap distance is measured and the substrate verticalposition is adjusted to obtain the requested air gap distance. The airgap at each point is evaluated until the gap distance is within anacceptable tolerance for performing the next process step.

The system then begins course alignment of the substrate 203 to thereticle 201. This initial fine alignment is performed as follows. Aminimum of three points along the substrate edge are scanned over theair gauge sensor 205. At each point, the (X, Y) position where the gapdistance dramatically changes (indicating the edge of the substrate) isrecorded and the data is processed to calculate an X/Y/yaw offset. Thereticle stage 602 is disconnected from the substrate stage 608. Thesubstrate stage is moved by the required offset distance calculate aboveand the reticle stage is then reconnected to the substrate stage.

The system then proceeds to perform final leveling and focusing of thesubstrate. This activity is performed in a nearly identical way to theinitial substrate leveling and focus process described above. The onlyexception being that all points are evaluated until the gap distance iswithin an acceptable tolerance specified by the user.

The system then begins fine alignment of the reticle to the substrate ifthe substrate has an existing image. Alignment is performed in thefollowing manner. The stage 116 with the substrate 203 and reticle 201is moved to the first of at least two positions so that the reticle keyis aligned to the alignment system (see FIG. 5a) which is in the centerof the optical field shown in FIG. 3c. This through-the-lens alignmentsystem then quantifies the misalignment between the reticle andsubstrate. The substrate and reticle are then moved to the secondalignment position where the process is repeated. After all positionshave been evaluated, the reticle stage 602 is locked to the platen 114and disconnected from the substrate stage 608. The substrate stage ismoved by an amount determined from the misalignment data. The reticleand substrate stages are then reconnected and the reticle stage isreleased from the platen. If necessary, the entire process can berepeated until misalignment is reduced to a level acceptable to theuser. If the system is unable to identify alignment geometries, a videodisplay 101 of the alignment fiducial images is presented to the userfor manual alignment.

With alignment complete (if necessary), the actual lithography processmay begin. The lithography is performed as follows. The stage 116(reticle stage 602 and substrate stage 608 are rigidly coupled togetherat this point) is moved to a position such that the illumination isblocked by the reticle stage in the right back corner. The illuminationis turned on by opening the exposure shutter. The stage accelerates to aconstant velocity (determined by user/process) in the -Y direction. Whenthe printable area of the reticle passes through the illumination field,the image is relayed to the substrate through lens system 117. It isimportant to note here that the lens system preserves image position orthat the image is said to be erect and non-inverted. Constant stagevelocity is maintained until the printable area of the reticle is out ofthe illumination field. At the end of the scan, the stage de-acceleratesto a stop, then indexes left (-X) by an amount determined by theillumination geometry. The stage repeats the previous process moving in-Y. The entire process above is repeated until the entire printable areaof the reticle has been completely scanned (exposed). This process isdone with the Sawyer type stepper motors to obviate the need for ametrology system on the stage because stepper motors can be commanded toan acceptably precise position without requiring a metrology system.

FIGS. 9a to 9g illustrate operation of a computer program which may beused in operation of the system according to one embodiment of theinvention. FIG. 9a provides an overall description of the systemsoftware. At step 902 job data are loaded from storage such as a 3.5"floppy disc, a network, or the like. At step 904 the reticle is loaded.At step 906 the substrate cassettes are loaded. At step 908 the systemdetermines if there is another substrate to process. When there aresubstrates to process, at step 910 the system will load a substrate ontothe substrate chuck. At step 912 the substrate will be processed and,then at step 914 the substrate will be removed from the chuck. Thesystem then again checks to see if there are more substrates in thecassette to process.

When there are no additional cassettes to process, the system removesthe cassette at step 916. At step 918 the system determines if there aremore cassettes to process, and additional cassettes are processedbeginning again at step 906. When there are no additional cassettes toprocess, the system terminates at step 920.

FIG. 9b illustrates the step 910 of performing loading of the substrateonto the substrate chuck in greater detail. At step 926 the systemdetermines if there is a substrate on the load articulator. If not, asubstrate is loaded from the cassette at step 928. At step 930, thesystem determines if the substate on the load articulator is prealignedand, if not, the substrate is aligned at step 932. At step 934, thesystem then determines if there is a substrate on the substrate chuck,and if not, transfers the substrate from the load articulator to thestage substrate chuck at step 936. If there is a substrate on the chuck,the system determines if processing of the substrate is complete, andwhen it is, removes the substrate from the stage, then proceeds to step936. Substrate loading is then complete.

FIG. 9c illustrates the substrate processing step 912 in greater detail.At step 942 the system performs the initial leveling and focusing of thesubstrate to the lens. At step 944 the system performs mechanicalalignment of the substrate to the lens. At step 946 final leveling andfocusing of the substrate to the lens is performed. At step 948 thesystem determines if the substrate and reticle need to be aligned and,if so, the substrate and reticle are aligned at step 950. The systemthen exposes the substrate at step 952, completing the process at step954.

FIG. 9d illustrates the process of leveling and focusing of thesubstrate and lens as shown in step 942 in greater detail. At step 956the system loads a list of all points to be considered when leveling andfocusing. At step 958 a flag is set for focusing. At step 960 the systemdetermines if more points are to be considered and, if so, at step 964the system moves the desired substrate focus location over the focus airgap sensor. At step 966 the system evaluates the substrate/lens air gapand at step 968 determines the adjustment needed for the desired airgap. At step 970 the system determines if the gap is in tolerance rangesand, at step 972, if not, adjusts the three focus motors as needed. Thesubstrate focus flag is then cleared, and the system returns to step 960until all points have been corrected. Thereafter, at step 962, it isdetermined if the substrate focus flag has been set and, when it has(i.e., when a substrate has been adjusted) the system is completed.

FIG. 9e illustrates the process for performing mechanical alignment ofthe substrate to the lens. At step 976 the system determines if morepoints along the substrate are to be evaluated. When there are, at step978 the new point is moved over the focus air gap sensor, and at step980 the edge is scanned to find transition and record position. Thesystem then returns to step 976 until no additional points are to beevaluated.

Thereafter, at step 982 the system calculates the required offset foralignment of the substrate to the lens. At step 984 the system isolatesthe reticle and the substrate stages. At step 986 the substrate stage ismoved by a necessary offset, and at step 988 the reticle and substratestages are reconnected. The system is then completed.

FIG. 9f illustrates the process for locking the reticle stage to thesubstrate stage at step 988 in greater detail. At step 992 the systemstops the air flow to the stage coupler air bearings. At step 994 thesystem applies em forces to the stage couplers. At step 996 the systemwaits for a period to secure the reticle to substrate stages. At step998 the system pulses the reticle stage platen electromagnets to ensurecontact, and at step 999 applies air to the reticle stage platen airbearings. The process of securing the substrate is then complete.

FIG. 9g illustrates the process 984 of unlocking the reticle and thesubstrate stage in greater detail. At step 901 the system pulses thereticle stage electromagnets. At step 903 the system removes air fromthe reticle stage air bearings. At step 905 the system waits for thereticle stage to be grounded to the platen, and at step 907 the systemremoves the em attractive forces on the stage couplers. At step 909 thesystem supplies air to the stage coupler air bearings. The process ofdecoupling is then complete.

FIG. 10a shows how the trapezoidal (a one-dimensionally symmetric shape)illumination field is overlapped during two adjacent sequential scans1002 and 1004 in the serpentine scanning process. The overlappingportions of adjacent scans adds to produce a substantially constant doseequivalent to the non-overlapped portion as seen in FIG. 10b. Thisprocess provides several benefits over existing lithography techniques.They are:

1. The ability to expose a large area with a small and less complex lenssystem.

2. Averaging of exposure across the Y axis, and X axis averaging forthose portions of the field that are overlapping in X.

3. Averaging of field distortion (including printable defects) acrossthe Y axis, and X axis averaging for those portions of the field thatare overlapping in X.

4. Because of the X overlap during the scanning operation, no stagemetrology is required during exposure.

5. The overlapping scan also eliminates the need for the stitchingtechnique employed by lithography machines that attempt to create imageslarger than their field size.

A second mode of substrate exposure is possible with the systemconfiguration described. In this second mode, the system can effectivelystep to any given location and align (if targeting was provided) andexpose an area described by the trapezoidal field shape. This behavioris identical to that class of lithography equipment known as "Step andRepeat."

Illumination intensity is always held constant by monitoring lightintensity in the illuminator 118 by sensor 331, and adjusting the lamppower supply 119. With the lithography process complete, the substratewill be moved into the unload cassette. The unload process is asfollows. The stage moves to the unload position at the front left of themachine and the pop-up chuck 613 vacuum is turned on. The pop-up israised until vacuum at the pop-up chuck is sensed. The stage substratevacuum is released and the articulator fork 110 is lowered to anelevation just below substrate. The unload articulator fork vacuum isturned on. The stage moves toward the front of the machine until thesubstrate is centered on the unload articulator fork 110. The unloadarticulator fork then raises until vacuum is sensed. Vacuum is thenreleased from the stage pop-up chuck. The stage moves toward the rear ofthe machine to clear the substrate. The unload articulator fork moves inthe +Z direction to an elevation required to deposit the substrate intoone of the unload cassette slots 103. The unload articulator then movesto the rear of the machine and thus moves the substrate into theappropriate unload cassette slot. Vacuum is removed from the unloadarticulator fork. The unload articulator fork moves down -Z, then towardthe front of the machine to clear the unload cassette 103.

IX. Conclusion

The system has a wide variety of desirable features, and allows for theready incorporation of additional features. For example, the system hasa built-in, integrated user interface which can be configured for aremote file server. The architecture of the system enables constructionof a system with a weight of less than 1000 lbs, preferably less than700 lbs, and most preferably less than 500 lbs, all with a relativelysmall footprint. The above system provides a substantially improvedlithography system and method.

The above description is illustrative and not restrictive. Manyvariations of the invention will become apparent to those of skill inthe art upon review of this disclosure. Merely by way of exampledifferent materials of construction may be utilized, and the variousoptical operations may be performed by holograms or other opticalelements. The scope of the invention should, therefore, be determinednot with reference to the above description, but instead should bedetermined with reference to the appended claims along with their fullscope of equivalents.

What is claimed is:
 1. A photolithographic process for manufacturingelectronic devices comprising the steps of:substantially locking areticle mount and a substrate mount together throughout thephotolithographic process, the reticle mount for holding a reticle andthe substrate mount for holding a substrate; and providing relativemotion of the interlocked reticle mount and substrate mount with respectto an optical relay system and an illumination system in multiple,adjacent, partially overlapping scans wherein the interlocked reticlemount and substrate mount are substantially fixed relative to each otherthroughout the providing step.
 2. The photolithographic process of claim1 wherein the providing step comprises moving the interlocked reticleand substrate mounts with respect to non-moving optical relay andillumination systems.
 3. The photolithographic process of claim 1wherein the providing step comprises moving the optical relay andillumination systems with respect to non-moving, interlocked reticle andsubstrate mounts.
 4. The photolithographic process of claim 1 furthercomprising the step of positioning a reticle on the reticle mount and asubstrate on the substrate mount such that the reticle and the substrateare oriented horizontally with respect to the local gravity vector. 5.The photolithographic process of claim 1 further comprising the step ofpositioning a reticle on the reticle mount and a substrate on thesubstrate mount such that the reticle and the substrate are orientedsubstantially vertically with respect to the local gravity vector. 6.The photolithographic process of claim 5 wherein the reticle andsubstrate are suspended from the reticle and substrate mounts,respectively.
 7. The photolithographic process of claim 1 wherein theproviding step comprises propelling the optical relay and illuminationsystems by a planar drive means.
 8. The photolithographic process ofclaim 7 wherein the planar drive means comprises a Sawyer type steppermotor.
 9. The photolithographic process of claim 1 further comprisingthe steps of:illuminating an alignment fiducial on the substrate, theillumination of the alignment fiducial being non-actinic and dark-field;and transmitting an optical alignment signal from the substrate throughthe optical relay system and the reticle.
 10. The photolithographicprocess of claim 1 wherein the step of substantially locking the reticleand substrate mounts together comprises positioning the reticle andsubstrate on opposing sides of the optical relay system and adjacent theillumination system.
 11. The photolithographic process of claim 1wherein the reticle is substantially the same size as the substrate, andthe optical relay system has an imaging field size which is smaller thanthe pattern on the reticle.
 12. The photolithographic process of claim 1wherein an intensity distribution of an image formed by the opticalrelay system normal to the direction of the scans is characterized by across-section, the providing step being implemented such that the scansoverlap to an extent necessary to achieve a substantially uniformexposure of the substrate to the image.
 13. The photolithographicprocess of claim 12 wherein the cross-section is substantiallytrapezoidal.
 14. The photolithographic process of claim 1 furthercomprising the step of illuminating the reticle with a constantintensity arc lamp.
 15. The photolithographic process of claim 1 furthercomprising the step of providing air bearing support for the interlockedreticle and substrate mounts.